Bluetooth: Mesh: Fix enabling Node Identity advertising

The bt_mesh_provision() call results in the provisioning link state
being cleared, so link.conn will become NULL. Add code to store the
information of whether PB-GATT was used and use this information after
the call instead of relying on link.conn anymore at this point.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>